Reactor Theory (Neutron Characteristics)DOE-HDBK-1019/1-93PROMPT AND DELAYED NEUTRONSRev. 0Page 31NP-02On the other hand, the average generation time for the six delayed neutron groups is the totaltime from the birth of the fast neutron to the emission of the delayed neutron. Again, three timeintervals are involved: (a) the time it takes a fast neutron to slow down to thermal energy,(b) the time the thermal neutron exists prior to absorption, and (c) the average time from neutronabsorption to neutron emission by the six precursor groups. The average time for decay ofprecursors from uranium-235 is 12.5 seconds. The other terms in the delayed neutrongeneration time are insignificant when compared to this value, and the average delayed neutrongeneration time becomes 12.5 seconds.A neutron generation time in the range of 10 seconds to 10 seconds or faster could result in-4 -5very rapid power excursions, and control would not be possible without the dependence upondelayed neutrons to slow down the rate of the reaction. The average generation time, and hencethe rate that power can rise, is determined largely by the delayed neutron generation time. Thefollowing equation shows this mathematically.(2-12)Example:Assume a prompt neutron generation time for a particular reactor of 5 x 10 seconds and-5a delayed neutron generation time of 12.5 seconds. If is 0.0065, calculate the averagegeneration time.Solution:This example demonstrates the effect delayed neutrons have on the neutron generation time andthus reactor control. If a reactor were to be operated in a sustained chain reaction using onlyprompt neutrons (= 0), the generation time from the previous example would be about5 x 10 seconds. However, by operating the reactor such that a 0.0065 fraction of neutrons are-5delayed, the generation life time is extended to 0.0813 seconds, providing time for adequateoperator control. Therefore, although only a small fraction of the total neutron population,delayed neutrons are extremely important to the control and maintenance of a sustained fissionchain reaction.SummaryThe important information in this chapter is summarized on the following page.